Large dams represent complex social-ecological systems, perhaps the most complicated projects among large infrastructures. Nowadays, developing and developed countries consider large dams as a viable solution to provide low-cost energy production and flood control for agriculture production. However, the debate about dams is generally focused on technical arrangements, lacking of a holistic perspective of analysis, while their effects may be disruptive for a wider number of factors. The present paper proposes to study large dams within the theory of common-pool resources, focusing on the relation between water security and sustainability. The use of a social-ecological framework facilitates a dynamic analysis among different variables of large dams. What is more, it permits a cross-scale analysis, enabling one to understand the extreme complexity of social-ecological changes in a considered system. This research will focus on the Ethiopian large dam Gibe III, predicted to start functioning at the end of this year. It is already altering the downstream conditions of Omo River and Lake Turkana, shared by Ethiopia and Kenya, posing a threat to the livelihoods of thousand people. However, the current discussion about it still appears limited to technical solutions to the dam implementation. Arguing the opposite, the social-ecological framework enables one to include information sharing, climate change and collective-choice rules as important elements to be considered to bring the discussion at a broader level of understanding. From the analysis of Ethiopia, it is found that large dams cannot alone be the answer to water security if they are not connected to more vast social-economic reforms. The paper argues that the interpretation of large dams must be considered as part of the broader social, ecological and politico-economic situation, transcending from the mere local situation. The overall picture is not whether not to build them or not, as there is not a real choice, but how to foster instruments of analysis that preserve the environment and societies, while defeating poverty.